On Tuesday/Wednesday June 5/6, Earth will have the best seat — the only seat — for a great show: the Transit of Venus across the face of the Sun. This is a relatively rare event, and the next one won’t happen until December 10, 2117, so I’m guessing this will be the last time you’ll be able to see it.

I have a lot of info below, so I’ve broken it up into sections. Also, a special note: Fraser Cain, Pamela Gay, and I are hosting a live online video chat star party for the transit! That live video feed will be embedded here on the blog at the time of the transit, so if you read this blog — and you do — you’ll see it. I’ll have more info closer to the date.

Oh — you can click on the pictures here to embiggen them and get more info, too.

A transit is when one object in the sky passes in front of another. In this case, we’ll see Venus move across the Sun’s face. Think of it as a mini-eclipse.

Venus orbits the Sun closer in than we do, taking about 225 days to circle it once. We don’t see a transit every time, though, because its orbit is tilted slightly (by about 3°) to the Earth’s, so most of the time it passes near the Sun, but misses it*. Only when the planets align just right do we see an actual transit.

The geometry works out in a funny way. Transits come in pairs 8 years between events, but each pair is separated from the next two by more than a century. The last pair was 1874/1882. The next transit after that was in 2004. Now we have the second of our current pair, and then the next two won’t be until 2117/2125!

I saw the last transit in 2004, and it was pretty awesome. Venus was just a small dot, barely a disk, seen in silhouette against the Sun, but it was clearly not a sunspot even without magnification. I’ll note here you shouldn’t observe the Sun without eye protection! I have more about that in the Observing the Transit section below.

Historically, the transit was used to measure the size of the solar system. In the 1600s, the distances to the planets were only known in units of the Earth’s orbit. So Venus was 0.7 times as far from the Sun as Earth, and Jupiter was 5 times as far. But the actual size of Earth’s orbit wasn’t known! Before we had space probes and radar (which we can use to bounce signals off planets and measure their distance directly) it was hoped the transit of Venus would allow it to be measured. It worked, but the details are pretty cool and well worth a read. The story is actually told pretty well on the Wikipedia page.

Interestingly, as seen from Saturn in December 2012, Venus will transit the Sun as well, and astronomers are hoping to use Cassini to see if they can detect it. Also, in 2014, Earth will transit the Sun as seen from Jupiter! Hubble may be used to observe Jupiter at that time to see if it can be detected as well. [Thanks to BABloggee Garrett Curley for this info! You can read more about these space observations at Physics World – you have to register, but it’s free.]

*[Actually, because the Earth is moving around the Sun as well, after one complete orbit Venus still has to "catch up" to the Earth to line up with the Sun again. That’s called the synodic period, and it takes about 584 days. Still, the tilt of Venus’s orbit means it doesn’t always cross the Sun directly every synodic orbit.]

So I wanna see it! What do I do?

Because Venus is transiting the Sun, it has to be daytime during the transit for you to see it (duh). The transit occurs from about 22:00 June 5 to 05:00 June 6 (UTC; subtract 4 hours for Eastern US time). The exact time depends on where you are on Earth; NASA has a list of times for US cities and for the rest of the world.

For a more general overview, here’s a map of where the transit is visible:

Places in white see the whole event. Light grey areas see either the first part of the transit with the Sun setting during the event, or the last part of the transit because it’s already in progress during sunrise. Areas in dark grey don’t get to see it at all (sorry eastern South America and western Africa; but you can watch it online!). Since the transit ends at 01:00 Eastern US time, the Sun sets before the transit ends, and the farther west you go the more of the transit you’ll see.

If you look, what will you see? The Sun is 115 times wider than Venus, but Venus is much closer to us than the Sun is (about 42 million km versus 150 million — 25 versus 93 million miles). Because of that, Venus will look to be about 1/30th the Suns diameter, just big enough to see as a tiny disk by eye (but DON’T LOOK AT THE TRANSIT UNPROTECTED; see below).

This is an illustrative animation of what the 7 hour transit will look like. The Sun probably will have sunspots visible during the transit, but Venus will be darker and rounder than they are.

OBSERVING THE TRANSIT SAFELY IS INCREDIBLY IMPORTANT. While looking at the Sun won’t necessarily cause permanent or total blindness, it’s not a good idea, and you should NEVER look at the Sun through binoculars or a telescope unless they are outfitted properly. If you don’t know what you’re doing with astronomical equipment, the best bet is don’t do it.

However, there are many ways to observe the transit safely. In fact, observing the transit is pretty much the same as observing a solar eclipse, so the same rules apply. I have a list and links on a post I wrote about the May 2012 solar eclipse. Basically, filtered glasses specifically designed for eclipses are good (not just sunglasses, and don’t look through exposed film or mylar or anything like that unless again it’s specifically designed for looking at the Sun). You can use binoculars or a telescope to project the light from the Sun onto a piece of paper (though you can damage your optics that way). You can build a pinhole projector, but unlike an eclipse the shadow of Venus is small and may be hard to see that way. A flat mirror covered with foil can be used to project the Sun as well — this video shows you how. Again though the image may not be sharp enough to let you see Venus.

The safest way, of course, is to watch it online. The view’s better, too. But still, there’s nothing like seeing it with your own eyes. Just be careful if you do.

Below are links where you can watch the transit live online. My top suggestion is you watch our live video star party, where we’ll have experts and live views through amateur telescopes across the planet. I’ll embed the video stream on a new blog post at the date and time of the transit, or you can watch it on CosmoQuest. That will be hosted using Hangouts On Air on Google+. If you’re signed up for G+, circle Fraser Cain to be able to participate in the chat room, too.

The more live webcasts there are, the better; it’s likely to be cloudy in some spots, so you can try others to see if they have better weather. I’ll update this list as more links come in.

Check this out: there’s even a comic book/graphic novel about the transit made in India and available in six languages (including English)! I gave it a once-over and it’s very engaging and looks pretty accurate.

That’s it! Enjoy the transit! Remember, if you miss it, the next one won’t be for 105 years. You might get to see it, but it won’t be as much fun when you’re a disembodied head in a jar. So grab this chance now.

Local astronomy clubs are offering public viewing, some with star parties that extend into the night. For example, there are at least six such events in the greater Detroit area. Sky & Telescopes’s Clubs and Organizations link might get you hooked up with a local group.http://www.skyandtelescope.com/community/organizations
Check out their event calender.

@3. Chris. I plan on taking advantage of that myself. Luckily, I’ll be able to see this one too. Got my 8” Dob all ready with objective solar filter.

Incidentally, if you’re in the neighborhood, there’s a transit of Earth as seen from Mars on 11/10/2084. What’s awesome is that the Moon will transit as well alongside the Earth. Just to tide you over until 2117.

It’s great that there are live webcasts so everyone can see the event, but I encourage you to find a local group of astronomers at a nearby school, university, or museum, and see if you can watch it in person!
The NASA website Phil posts above has listings of tons of local events.
The group I’ll be joining is going to Easter Island for this: http://www.das.uchile.cl/~drodrigu/easter/index_en.html

About the Earth transiting the Sun, here’s the transit of both as seen from Mars on 11/10/2084. You can see the Moon down and to the left from the Earth. Note the different sizes and the angular distance between the two!

BA readers in the West Michigan / Northern Indiana region (or beyond) are invited to witness the Transit of Venus at Warren Dunes State Park, on the shore of Lake Michigan. The event is hosted by the Kalamazoo Astronomical Society. We’ll begin with a Solar Star Party (and more) at 4pm and continue until sunset. Please see our website for further details:

@Josie (#11):
“I’ve read that welder’s goggles are also ok for use in viewing the sun –is that true?”

The answer is “almost certainly not.”

The only welder’s glass that is safe is number 14 or darker (which is much darker than most welders use). Unfortunately, there are many who use lighter glass and assume, because they don’t see spots afterwards or feel any discomfort, that they are okay. NOT TRUE! Staring at the sun through a lighter glass will greatly increase the odds of cataracts later in life. Also, stacking two number 7 glasses (or an 8 and a 6, or…) together does not give the same protection as number 14. If the glass isn’t clearly marked with 14 or higher, DON’T USE IT!

Got everything ready. Pinhole camera viewer, baader solar filter for telescope, #14 welders glass, and a bunch of those disposable eclipse glasses. Now I just have to wait fo the weather to determine where we view from. If not locally then we head over the mountains and view from a rest stop on the highway.

@Pjb: Although it can be safe that way with all conditions just right, the usual practice is to auto-disclaimer and assume the conditions won’t be all right… otherwise, you’d get people staring when the sun is too high, staring when the sky is too clear… it’s safer just to say “don’t do it.”

The only time I was ever able to look at the sun comfortably and to see sunspots on it with my naked eye was right at sunset in Florida, facing westward across the Gulf of Mexico. You really need to see it right at the edge to get enough atmospheric dimming to make it safe, and even then there are probably horrible x-rays screwing up your retina for all I know. Other more knowledgeable people are on this site and I am sure they have more accurate info.

so MULTIPLE DISCLAIMERS… lol. Don’t do it (If you are at a completely flat horizon and you can see the sun without wincing, though, you’re probably ok for a few seconds.)

Still hoping telescopes.net ships my eclipse glasses soon. If I don’t have them on Saturday, I won’t be able to hand them out to friends.

@17 Jess: I was going to give a flip answer and say it’s no time at all before the oceans start to boil, but then I calculated that the stable temperature of a blackbody at that distance is less than 100 celsius. So it’s an interesting question, and I don’t know the answer.

The BadAstro says “Earth will have the best seat — the only seat — for a great show: the Transit of Venus across the face of the Sun. ” — Surely there are transits visible on other planets, if anyone were there to see them. Wouldn’t Transits of Earth (and Venus) occur on Mars (obviously at different times).

tcoreyb@22: X-rays are not the problem. The problem is the lens of your own eye focusing the intense visible (and maybe near IR) light onto your retina and killing it. Any sufficiently bright light source would also blind you. X-rays can’t be focused by your eye or any normal lens.

I watched it in 2004 (whatever year it was) at sunrise. I could see it with the naked eye (disclaimer) and projected with binoculars. I’m hoping the weather cooperates and I can watch it this time on the shores of Lake michigan.

Hmm, I calculated that the transit would take 4 hours. Back to the drawing board, I guess. But the good side is, this means we’ll have 4-5 hours of viewing time here in Colorado, instead of the 2-3 hours I was expecting. More time for clouds to clear, just in case.

I used to stare at the sun at sunset for a minute at a time when I was about 6 years old. I’m now 36, and while I can’t say my vision’s perfect, its only flaws are in focusing. Of course, I stared at the sun out of a contrarian attitude–if they’re not going to tell me why not to do it, I’m going to do it. There was also the logic that the sun is going to get in land animals’ eyes all the time, especially those without binocular vision, so how bad can it be?

I have a Mark 15 Davis sextant. It has three horizon shades, and four index shades. Each has a dark-gray filter (which I think is for polarization, useful for glare on the water), an orange one, and a blue one. The index shades also have an extra blue one. And I can’t tell if they are made of plastic or glass (but it’s probably the former since this is an inexpensive sextant).

Would these shades used together be sufficient to protect my eyes when viewing a transit or an eclipse? After all, you do need to see the sun when using a sextant.

In non-news, an Earth-mass planet, tentatively named Sol c, will be discovered transiting the nearby star of Sol at a distance between 0.5 and 1.0AU. Analysis of absorption spectra will indicate carbon dioxide and nitrogen in the atmosphere. As the planet lies within the Goldilocks zone, the presence of atmospheric organic compounds and the Earth-like mass raise exciting possibilities for life on this planet.

I’m curious- if the earth replaced Venus in the latter’s orbit, what would the temperature range go up to?

The same as Venus – the two are virtually twin planets and Venus probably started out very much like Earth with oceans and maybe even primitive life before the rising solar luminosity created a runaway greenhouse effect.

So put Earth where Venus is and it would ultimately end up almost exactly like Venus – which has a surface temperature of 480 degrees Celsius or 860 Fahrenheit* – and where’re eventually heading in that direction as our Sun slowly brightens over hundreds of millions of years. Before our Sun quite becomes a red giant or, I think, even a yellow sub-giant, Earth will no longer be habitable and will closely resemble our hellish sister planet.

Minds you, when Earth becomes like Venus I hate to think what Venus itself will be like!

How long would we have before the oceans started to boil?

I don’t know. It would certainly take time, won’t happen overnight but will definitely happen. Could be worth asking this question on the BAUT forum maybe?

Disagree. Welder’s glasses have to be designed specifically to block short-wave UV. If it’s safe to weld with them, it should be safe to view the Sun with them.

Well, you’re wrong. Dangerously so, IMO.

A welding arc has a pretty narrow spectrum of emissions, while the sun emits light as a black body (interestingly, “the sun” was the answer to a question on QI – “what is the blackest body in the solar system?”, because the sun’s emissions are pretty close to pure black-body radiation).

The damage caused by direct sunlight on the eye is not just from UV – it is mainly from the visible light, and only partly from UV and IR.

Think about it. The human eye has evolved to bring visible light to a sharp focus on the retina, and the cone cells of the retina have evolved to absorb a large fraction of that visible light. UV and IR do not come to such a sharp focus on the retina (although UV poses other hazards because it is more energetic than visible light).

@Noah Fect (#24):
“Welder’s glasses have to be designed specifically to block short-wave UV. If it’s safe to weld with them, it should be safe to view the Sun with them.”

This assumes that all sources of UV are the same, both in flux and spectral energy distribution (and fails to take IR into account whatsoever). If that was the case, why would there be different grades of welder’s glass? Your logic is flawed and dangerous, and I sincerely hope that no one damages their eyesight believing it.

Just set up binoculars on a steady support, plain white paper on the ground below and watch Venus make it’s transit across the sun that way. No pain, the dot is far too small to see it easily directly through filters over your eyes. Saw it in 2004, very moving.

RE: USING ELEMENTARY SCHOOL ECLIPSE DEVICES….Back in the 60’s, when the solr eclpse came to my science class, our teacher had us make “safe” boxes to watch. We used a shoe boxes with a small hole on one end and an opening to view thru. On one of the holes?, we taped a piece of old film negative…one that is all dark cause the pic didn’t develope, taped on the top and were cool to go…Tell us if this is a safe practice and plz refresh our overfull memories on how to make them, if it’s cool…

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